1. Field of the Invention
The present invention relates to a video processing apparatus, and more particularly to an apparatus and method for converting a resolution of a compressed video and for scaling down a resolution of the video contents.
2. Background of the Related Art
In general, mobile terminals include LCDs with a variety of different resolutions. Thus, video information to be displayed on the LCD must be converted to match the resolution of the LCD being used. Therefore, the resolution of video information often needs to be converted to a new resolution. Further, because the resolution of the displays in mobile terminals is generally less than a resolution of a video to be displayed, the resolution of the video information is down converted using a resolution converter, for example.
There are generally two types of resolution converters: one is an open-loop converter and the other is a closed-loop converter.
First, an example of an open-loop converter will be described with reference to FIG. 1. As shown, the open-loop converter includes a VLD (Variable Length Decoding) unit 11 for decoding a variable length of a video signal; a de-quantizer 12 for de-quantizing an output signal of the VLD unit 11; a down-sampling filter 13 for scaling down a resolution of a signal output from the de-quantizer 12; a quantizer 14 for quantizing an output signal of the down-sampling filter 13; a VLC (Variable Length Coding) unit 15 for coding a variable length of an output signal of the quantizer 14; and a motion vector re-sampler 16 for scaling down a motion vector of the input video signal. Note, a resolution scale-down ratio of the down-sampling filter 13 is the same as a motion vector scale-down ratio of the motion vector re-sampler 16.
The open-loop converter has a simple structure because a motion vector is not compensated when de-quantizing and quantizing the video signal. However, when continuously input P-frames are converted into a lower resolution, an error is accumulated and drifted, which degrades the picture quality. Namely, an error in one frame may not significantly affect the picture quality, but when the error is accumulated and drifted to other frames, the picture quality becomes more and more degraded. This type of problem is called an error drift problem.
The closed-loop converter is shown in FIG. 2 and is used to address the error drift problem of the open-loop converter shown in FIG. 1. As shown in FIG. 2, the closed-loop converter includes a decoding unit 19 for decoding an input video signal in DCT (Discrete Cosine Transform) units; a down-sampling filter 26 for scaling down a resolution of a signal output from the decoding unit 19; and a coding unit 20 for compensating a video signal output from the down-sampling filter 26 with a final motion vector and coding the compensated video signal.
The decoding unit 19 includes a VLD unit 21 for decoding a variable length of the video signal; a de-quantizer 22 for de-quantizing a signal output from the VLD unit 21; a first motion vector compensating unit 23 for compensating a motion vector using a motion vector input through the VLD unit 21; a first frame memory 24 for storing a signal restored as a DCT coefficient value by adding the motion vector compensated by the first motion vector compensating unit 23 and an output signal of the de-quantizer 22; and a motion vector re-sampler 25 for scaling down a motion vector of the video signal input through the VLD unit 21.
In addition, the first motion vector compensating unit 23 compensates a motion vector using a signal stored in the first memory 24 as a reference frame. The motion vector re-sampler 25 calculates a final motion vector (Vn) using a general known method (rather than estimating the motion vector).
Next, the operation of the closed-loop converter will be described. When an I-frame is input to the closed-loop converter, the frame is decoded in its variable length through the VLD unit 21 and then restored to DCT coefficients through the de-quantizer 22. The VLD unit 21 also extracts various supplementary information (e.g., header information, motion information, etc.) from the I-frame.
A signal corresponding to the restored DCT coefficient is Xnc. The Xnc signal is input to the down-sampling filter 26 and becomes an Xn signal with a scaled-down resolution. Further, the Xn signal is then compressed in the coding unit 20. The Xnc signal for the I-frame is also stored in the first frame memory 24, and is used as a reference frame for compensating a motion of a P-frame input after the I-frame.
Further, when the P-frame is input to the closed-loop converter, a result value obtained by performing a motion compensation on the P-frame based on the DCT coefficient value of a previous frame stored in the first frame memory 24 and an error signal Enc of the P-frame output through the VLD unit 21 and the de-quantizer 22 are added to restore the DCT coefficient value Xnc. The restored DCT coefficient value Xnc is then input to the down-sampling filter 26 and becomes the Xn signal with the scaled-down resolution. The Xn signal then compressed in the coding unit 20.
As mentioned above, the closed-loop converter reduces the error drift by compensating the motion vector. However, the structure of the closed-loop converter is more complex than the open-loop converter.